This invention relates to load indicating members and, more particularly, to load indicating members, such as fasteners, having ultrasonic transducers.
In many operations, it is desirable to determine the amount of longitudinal load experienced by a longitudinally stressed member. This information is particularly useful when the longitudinally stressed member is a fastener since the measurement of longitudinal stress provides a verification of the existence of a proper joint.
Many prior art techniques have been developed to indicate the amount of longitudinal stress experienced by a fastener by providing a load indicating feature to the fastener itself. This is usually done by interconnecting one end of an elongated member, such as a pin, to a portion of a fastener. While each of the various pin-type load indicating members and load measuring devices known in the art provides its own advantages in terms of accuracy, ease of manufacture, or ease of reading, they are still expensive to manufacture, since they each require extensive modifications and the addition of a centrally located pin-member. As a result, such load indicating members are only selectively used in practice, either where there is a specific immediate diagnostic need or where there is a serious recognized safety hazard involved. These members are simply too expensive for routine use in assemblies which may only occasionally benefit from such monitoring.
An alternative approach to measuring the elongation of a member or fastener is to use an ultrasonic measurement device. Typically, this is done by removably interconnecting an ultrasonic transducer to one end of the fastener being measured, usually the head of a bolt. In order to obtain a reliable indication, the head of the bolt must be ground extremely flat and a reliable ultrasonic transmission media must be applied to the head of the bolt. The transducer must be properly positioned on the bolt and held in position while the measurements are being taken. Various examples of techniques and apparatuses using this method are known in the art. Furthermore, the prior art teachings include the notion of combining the measuring device with a tightening tool so that the information gained from measuring the elongation of the bolt can be used for determining when to shut off the tool or, alternatively, for monitoring the tightening process to determine whether a proper joint has been formed.
While the above-mentioned products and apparatuses can provide reliable information about the fastener and joint, they are in very limited use. This is mainly because the bolt must be carefully manufactured and must be easily accessible to the instrumentation. Thus, ultrasonic tension measurement is recognized as a highly accurate laboratory tightening method for calibration, application testing and for tightening very critical joints. It is replacing strain gage bolts in several calibration and critical quality control applications. However, practical difficulties associated with taking ultrasonic tension measurements have prevented its application as a general assembly tightening strategy. These practical difficulties include: difficulty in maintaining reliable acoustic coupling during tightening; difficulties presented by equipment expense and complexity; and difficulties presented by experimental determination of parameters for each joint.
Some attempts have been made to overcome the above-mentioned difficulties by incorporating a piezoelectric or other ultrasonic transducer into the member itself. Examples of such members are disclosed, for example in U.S. Pat. No. 4,127,788 issued Nov. 28, 1978 to Daugherty and U.S. Pat. No. 4,294,122 issued Oct. 13, 1981 to Couchman. Each of these disclosures provides an instrumented load bearing fastener which has been modified to incorporate a stress indicating feature. However, like the pin-type fasteners described previously, these instrumented fasteners are greatly modified in order to accept large and complicated ultrasonic sensing devices. They are therefore prohibitively expensive for wide spread use.
U.S. Pat. No. 4,846,001, Kibblewhite, teaches the use of a thin piezoelectric polymer film sandwiched between two thin electrodes, which is permanently, mechanically and acoustically coupled to the upper surface of a member and is used to determine the length, tensile load, stress, or other tensile load dependent characteristic of the member by ultrasonic techniques. While the invention represents a significant advance over the prior state of the art in terms of performance, ease of manufacture and manufacturing cost, there are disadvantages with a transducer of this construction. These disadvantages relate to environmental performance, in particular the maximum temperature limitations of the polymer material which restricts its application, and the possibility of the transducer, fixed to the fastener with adhesive, coming loose and causing an obstruction in or damage to a critical assembly.
Most stressed members, such as fasteners, for example, are subject to varying stresses along the length of the member. It is therefore often desirable to determine the stress in a specific part of the member. The use of load indicating members incorporating artificial reflectors provides a means of measuring a load dependent characteristic over a specific part of the member. U.S. Pat. No. 4,569,229, de Halleux, teaches of a method of manufacturing and measuring stress in a member incorporating artificial ultrasonic reflectors. Steblay, U.S. Pat. No. 4,601,207 discloses a mine roof bolt and a means of measuring the strain in a mine roof bolt incorporating an artificial reflector, wherein the artificial reflector is a hole drilled radially through the bolt at a predetermined distance from the head of the bolt.
All the above-mentioned ultrasonic methods of determining stress in a load indicating member require a zero load measurement in addition to the measurement taken under the desired loaded condition in order to determine the absolute load in the member. Furthermore, all use a direct or indirect measurement of the out and return time of flight of a longitudinal ultrasonic wave. Holt, U.S. Pat. No. 4,602,511, teaches of a method which uses the times of flight of both longitudinal and transverse waves to determine the stress in a member without taking a zero measurement. This is desirable in the measurement of tensile load in previously installed fasteners, for example.
The use of transverse ultrasonic waves, however, requires both a transducer capable of generating transverse waves and an acoustic coupling media capable of transmitting transverse waves into the member. Special acoustic couplants are required with temporarily attached transducers, since transverse waves cannot generally be transmitted through liquids. Although adhesives can transmit transverse ultrasonic waves, generation of transverse waves using the polymer film transducers, disclosed by Kibblewhite in U.S. Pat. No. 4,846,001, has not been demonstrated.
What is desired, therefore, is an ultrasonic transducer permanently attached to a fastener, to provide accurate tightening information during assembly, which can not come loose and cause an obstruction in or damage to a critical assembly.
What is secondly desired is such ultrasonic transducer permanently attached to a fastener which can withstand the operating environment and, in particular, the operating temperature of the fastener, so that the fastener can be reused or, so that the load in the fastener can be measured periodically during the operation of the assembly in which the load indicating fastener is installed.
What is further desired is such ultrasonic transducer permanently attached to a fastener which is capable of transmitting both longitudinal and transverse waves for the measurement of stress in already installed fasteners.
What is also desired is such ultrasonic transducer permanently attached to a fastener which can be manufactured to direct ultrasonic waves toward artifical reflectors within the fastener.
What is further desired is such ultrasonic transducer permanently attached to a fastener which can be manufactured to generate high frequency ultrasonic waves, 10-500 MHz, for example, to provide improved accuracy in the measurement of ultrasonic out and return times of flight.
What is additionally desired is such ultrasonic transducer permanently attached to a fastener which can be manufactured to generate high frequency ultrasonic waves, 10-500 MHz, for example, to provide improved resolution for the detection of small ultrasonic artifical reflectors or small manufacturing defects.
What is even further desired is such ultrasonic transducer permanently attached to a fastener which does not require a separate high voltage polarization operation as part of its manufacturing process.
What is further additionally desired is such ultrasonic transducer permanently attached to a fastener which can be manufactured at low cost using high volume manufacturing methods.